1. Field of the Invention
This invention relates to a process for the preparation of cinnamic acid esters. Cinnamic acid derivatives are useful as antioxidants and ultraviolet absorbers in plastics. They are also useful as intermediates in the synthesis of medicines, agricultural chemicals and perfumes.
2. Background Information
A process for preparing a cinnamic acid ester by the condensation of a benzaldehyde with an acetic acid ester is known as the Claisen-Schmidt reaction [Ber., 23, 976 (1890)]. When the Claisen-Schmidt reaction is effected in the presence of sodium, however, the Cannizzaro reaction occurs preferentially, to give benzyl alcohol as the main product and to afford a cinnamic acid ester, which is the desired product, in a low yield.
German Patent DE709227 describes a process for preparing ethyl cinnamate or methyl cinnamate by a condensation reaction between a benzaldehyde and an ethyl or methyl acetate in the presence of sodium hydride as a base. Sodium hydride, however, is not easy to handle and is expensive and so that there is a demand to improve this process.
In Org. Synth. Coll., Vol. 1, 252, a process for preparing ethyl cinnamate by condensing benzaldehyde and ethyl acetate in the presence of a small amount of ethanol using finely dispersed sodium as a base is described. This procedure is not industrially acceptable, because sodium cannot easily be handled and besides, the yield of this process is not sufficiently high, specifically, it is generally between 68 and 74%.
Japanese Patent Application Kokai Sho 61-7236 describes a process for preparing a cinnamic acid ester by a condensation reaction between a benzaldehyde and an acetic acid ester, using an alcoholic solution of a metal alkoxide as a base. This procedure, however, requires a specific purification step, such as distillation, in order to isolate the cinnamic acid ester from the reaction mixture, because 3-methoxy-3-phenylpropionic acid is produced as a by-product in an amount of between 4.7 and 12.5%. This procedure is therefore associated with several problems, such as lower yields, troublesome purification and treatment of by-products.
U.S. Pat. No. 4,618,698 describes a process for the preparation of an optionally substituted cinnamic acid by reacting an optionally substituted benzaldehyde with an optionally substituted acetic acid ester with an alcoholate in the presence of an alcohol to form a mixture of an optionally substituted cinnamic acid ester and a .beta.-alkoxy-.beta.-phenylpropionic acid, and then hydrolysing the mixture under acidic or basic conditions to give an optionally substituted cinnamic acid. This optionally substituted cinnamic acid then needs to be esterified to give the desired optionally substituted cinnamic acid ester.
U.S. Pat. No. 5,359,122 describes a process for preparing, inter alia, an optionally substituted cinnamic acid or ester thereof by reacting a dialkyl acetal of an aromatic aldehyde with ketene in the presence of a catalytic amount of a protic or Lewis acid to give a 3-arylpropionic acid derivative, which is then reacted with an acid or base in the presence of an alkanol to give the desired optionally substituted cinnamic acid or ester.